drm/i915: enable and disable DDI_FUNC_CTL at the right time

And the right time is exactly after/before changing PIPE_CONF. See the
documentation about the mode set sequence.

This code is not inside any encoder-specific callback because
DDI_FUNC_CTL is part of the pipe, so it is used by all encoders.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>